Questions raised about oldest mammal.

Headlines earlier this month trumpeted the discovery of the oldest evidence of animal life, but new research challenges the reported age of these fossils from central India.

The specimens in question are a series of squiggly grooves in sandstone dated to be 1.1 billion years old. Adolf Seilacher of Yale University and the University of Tubingen in Germany and his colleagues last year described these marks as fossilized worm burrows, making them nearly twice as ancient as the oldest animal remains (SN: 11/1/97, p. 287). They formally reported their findings in the Oct. 2 SCIENCE.

If confirmed, the purported worm tracings would eat holes through ideas about animal evolution. An Indian scientist, however, has found evidence that the worm tracks could be far younger, according to Martin D. Brasier of Oxford University in England, who wrote a commentary in the Oct. 8 NATURE. R.J. Azmi of the Wadia Institute of Himalayan Geology in Dehra Dun, India, has found shelly fossils of animals in limestone above the sandstone containing the worm impressions. Scientists had formerly dated both the limestone and sandstone to be about 1.1 billion years old, but the shells in the limestone indicate that this layer is only about 540 million years old.

That opens the possibility that the underlying sandstone formed at about the same time, when worm burrows were quite common, according to Brasier. On the other hand, he says, there could be a big gap in age between the sandstone and the limestone above it, which would make the tracks much older than 540 million years. The difference would affect the standing of Brasier's own discovery. In January, he and a colleague reported finding 600-million-year-old fossil worm tracks, the oldest known prior to Seilacher's report.

Samuel A. Bowring, a geochronologist at the Massachusetts Institute of Technology, offers a way to reconcile the relatively young fossils found within rocks that appear quite old. When scientists dated the Indian deposits, they may have calculated the age of 1.1-billion-year-old rock grains that washed into the ocean and became incorporated in much younger limestone and sandstone. Further dating work is needed, he says. "Whenever we find fossils like this that seem to rewrite the book, it's critical to have unequivocally precise ages."